Dehydrogenative polymerization of conifery alcohol on macromolecular lignin templates

Abstract

Lignins, the aromatic biopolymers present in all vascular plant cell walls, may embody as many as 10 different linkages between the constituent ( p-hydroxyphenyl)propane residues. The final step in lignin biosynthesis has traditionally been thought to involve the random coupling of radical intermediates produced during the dehydrogenative polymerization of monolignol precursors. However, the enzyme-catalysed dehydropolymerization of coniferyl alcohol in homogeneous solution has now been found to be strongly affected by small quantities of macromolecular lignin components. Under these circumstances, when the radical concentrations remained low, the molecular weight distributions of the dehydropolymerisates formed with horseradish peroxidase and H 2O 2 exhibited dramatic increases in the populations of the largest species. The phenomenon was not suppressed by prior methylation of the aromatic hydroxyl groups on the causative lignin macromolecules—which, therefore, were able to promote the formation of large covalent species from the monolignol without direct participation in radical coupling. If operative in lignifying tissues, such an effect could provide the organization necessary for replicating sequences of interunit linkages along macromolecular lignin chains. In this regard, the enzyme-catalysed dehydropolymerization of coniferyl alcohol would display the character of a typical template polymerization reaction. © 1997 Elsevier Science Ltd. All rights reserved